Self-adhesive composite reinforcement

ABSTRACT

Composite reinforcement (R- 2 ) that is self-adhesive by curing to a diene rubber matrix, which can be used as reinforcing element for a tire, comprising: at least one reinforcing thread ( 20 ), for example a carbon steel cord; a first layer ( 21 ) of a thermoplastic polymer, the glass transition temperature of which is positive, for example 6,6 polyamide, covering said thread; and a second layer ( 22 ) comprising an unsaturated thermoplastic stirene elastomer, the glass transition temperature of which is negative, for example an SBS (stirene-butadiene-stirene) copolymer, covering the first layer ( 21 ). Process for manufacturing such a composite reinforcement and rubber article or semi-finished product, especially a tire, incorporating such a composite reinforcement.

RELATED APPLICATION

This is a U.S. National Phase Application under 35 USC 371 ofInternational Application PCT/EP2010/053155 filed on Mar. 12, 2010.

This application claims the priority of French patent application no.09/51782 filed Mar. 20, 2009, the entire content of which is herebyincorporated by reference.

FIELD OF THE INVENTION

The field of the present invention is that of reinforcing elements orreinforcements, notably metallic ones, which can be used to reinforcediene rubber articles or semi-finished products such as, for example,pneumatic tires.

The present invention relates more particularly to reinforcements of thehybrid or composite type that consist of at least one core, inparticular a metal core, said core being sheathed or covered by one ormore layers of a thermoplastic material.

BACKGROUND OF THE INVENTION

The sheathing of metallic reinforcements with thermoplastic materials,such as for example a polyamide or polyester, has been known for a verylong time, especially so as to protect these reinforcements from varioustypes of external attack such as oxidation or abrasion, or else for thepurpose of structurally stiffening, by bonding them together, variousgroups of threads or thread assemblies such as cords, and thusincreasing particularly their buckling resistance.

Such composite reinforcements, together with their use in rubberarticles such as pneumatic tires, have been described in many patentdocuments.

Patent application EP 0 962 576 has for example described areinforcement, made of steel or aramid textile, sheathed by athermoplastic material such as a polyester or polyamide, for the purposeof improving its abrasion resistance.

Patent application FR 2 601 293 has described the sheathing of a metalcord with a polyamide so as to use it as a bead wire in a pneumatic tirebead, this sheathing advantageously enabling the shape of this bead wireto adapt to the structure and to the operating conditions of the bead ofthe tire that it reinforces.

Patent documents FR 2 576 247 and U.S. Pat. No. 4,754,794 have alsodescribed metal cords or threads that can be used as a bead wire in atire bead, these threads or cords being doubly-sheathed or eventriply-sheathed by two or even three different thermoplastic materials(e.g. polyamides) having different melting points, with the purpose, onthe one hand, of controlling the distance between these threads or cordsand, on the other hand, of eliminating the risk of wear by rubbing or ofcorrosion, in order to use them as a bead wire in a pneumatic tire bead.

These reinforcements thus sheathed with a polyester or polyamidematerial have, apart from the aforementioned advantages of corrosionresistance, abrasion resistance and structural rigidity, the notinsignificant advantage of them being able to be subsequently bonded todiene rubber matrices using simple textile adhesives called RFL(resorcinol-formaldehyde-latex) adhesives comprising at least one dieneelastomer, such as natural rubber, which adhesives are known to providesatisfactory adhesion between textile fibres, such as polyester orpolyamide fibres, and a diene rubber.

Thus, it may be advantageous to use metal reinforcements not coated withadhesive metal layers, such as with brass, and also surrounding rubbermatrices containing no metal salts, such as cobalt salts, which arenecessary as is known for maintaining the adhesive properties over thecourse of time but which significantly increase, on the one hand, thecost of the rubber matrices themselves and, on the other hand, theiroxidation and aging sensitivity (see for example the patent applicationWO 2005/113666).

However, the above RFL adhesives are not without drawbacks: inparticular they contain as base substance formaldehyde, a substancewhich it is desirable long-term to eliminate from adhesive compositionsbecause of the recent changes in European regulations regarding thistype of product.

Thus, designers of diene rubber articles, especially tire manufacturers,are presently seeking new adhesive systems or new reinforcements thatenable all or some of the aforementioned drawbacks to be alleviated.

BRIEF DESCRIPTION OF THE INVENTION

One object of the invention is to provide a novel compositereinforcement which requires no sizing treatment in order to adhere torubber, in a certain sense of the type self-adhesive by curing, andwhich consequently enables the above objective to be achieved.

One aspect of the invention is directed to a composite reinforcementcomprising:

-   -   at least one reinforcing thread;    -   a first layer of a thermoplastic polymer, the glass transition        temperature of which is positive, covering said thread; and    -   a second layer comprising an unsaturated thermoplastic stirene        elastomer, the glass transition temperature of which is        negative, covering the first layer.

Unexpectedly, it has been found that the presence of this unsaturatedthermoplastic stirene elastomer makes it possible to ensure that thecomposite reinforcement of the invention adheres directly and stronglyto a diene elastomer matrix or composition such as those widely used intires.

Another aspect of the invention is directed to a process formanufacturing the above composite reinforcement, said process comprisingat least the following steps:

-   -   the reinforcing thread is covered by a layer of the        thermoplastic polymer having a positive glass transition        temperature;    -   a second layer comprising the unsaturated thermoplastic stirene        elastomer, having a negative glass transition temperature, is        deposited on the thread thus covered; and    -   the assembly undergoes a thermo-oxidative treatment in order to        bond the two layers together.

A composite reinforcement according to an embodiment of the inventioncan be used as a reinforcing element for rubber articles orsemi-finished products, particularly tires, especially those intended tobe fitted onto motor vehicles of the passenger type, SUVs (“SportUtility Vehicles”), two-wheel vehicles (especially bicycles andmotorcycles), aircraft, or industrial vehicles chosen from vans, “heavy”vehicles, i.e. underground trains, buses, road transport vehicles(lorries, tractors, trailers), off-road vehicles, such as agriculturalor civil engineering machines, and other transport or handling vehicles.

Another aspect of the invention relates to any rubber article orsemi-finished product, in particular a tire, that includes a compositereinforcement according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows, in cross section, an example of a composite reinforcementaccording to the invention;

FIG. 2 shows, in cross section, another example of a reinforcementaccording to an embodiment of the invention;

FIG. 3 shows, in cross section, another example of a reinforcementaccording to an embodiment of the invention;

FIG. 4 shows, in cross section, another example of a reinforcementaccording to an embodiment of the invention; and

FIG. 5 shows, in radial section, a tire having a radial carcassreinforcing member, in accordance with the invention, incorporating acomposite reinforcement according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

In the present description, unless expressly indicated otherwise, allthe percentages (%) indicated are % by weight.

Moreover, any range of values denoted by the expression “between a andb” represents the range of values starting from more than a to less thanb (i.e. with the limits a and b excluded), whereas any range of valuesdenoted by the expression “from a to b” means the range of valuesstarting from a and going up to b (i.e. including the strict limits aand b).

The composite reinforcement of the invention, capable of adheringdirectly to an unsaturated rubber composition and able to be used inparticular for reinforcing diene rubber articles, such as tires,therefore has the feature of comprising:

-   -   at least one reinforcing thread (i.e. one or more reinforcing        threads);    -   a first layer of a thermoplastic polymer, the glass transition        temperature (denoted hereafter by Tg₁) of which is positive        (i.e. above 0° C.), covering said thread (or said threads); and    -   a second layer of a thermoplastic stirene (TPS) elastomer of the        unsaturated type, the glass transition temperature (denoted        hereafter by Tg₂) of which is negative (i.e. below 0° C.),        covering said first layer.

In other words, the composite reinforcement of the invention comprises asingle reinforcing yarn or several reinforcing yarns, each reinforcingyarn being covered by two different superposed layers of thermoplasticpolymers. The structure of the reinforcement of the invention isdescribed in detail below.

In the present application, the term “reinforcing thread” is understoodin general to mean any elongate element of great length relative to itscross section, whatever the shape, for example circular, oblong,rectangular, square, or even flat, of this cross section, it beingpossible for this thread to be straight or not straight, for exampletwisted or wavy.

This reinforcing thread may take any known form. For example, it may bean individual monofilament of large diameter (for example and preferablyequal to or greater than 50 μm), an individual ribbon, a multifilamentfibre (consisting of a plurality of individual filaments of smalldiameter, typically less than 30 μm), a textile folded yarn formed fromseveral fibres twisted together, a textile or metal cord formed fromseveral fibres or monofilaments cabled or twisted together, or else aband or strip comprising several of these monofilaments, fibres, foldedyarns or cords grouped together, for example aligned along a maindirection, whether straight or not.

The or each reinforcing thread has a diameter preferably smaller than 5mm, especially in the range from 0.1 to 2 mm.

Preferably, the reinforcing thread is a metal reinforcing thread,especially a carbon steel wire such as those used in steel cords fortires. However, it is of course possible to use other types of steel,for example stainless steel. When a carbon steel is used, its carboncontent is preferably between 0.4% and 1.2%, especially between 0.5% and1.1%. The invention applies in particular to any steel of the steel cordtype having a standard or NT (“Normal Tensile”) strength, a high or HT(“High Tensile”) strength, a very high or SHT (“Super High Tensile”)strength or an ultra-high or UHT (“Ultra High Tensile”) strength.

The steel could be coated with an adhesive layer, such as a layer ofbrass or zinc. However, advantageously a bright, i.e. uncoated, steelmay be used. Furthermore, by virtue of the invention, the rubbercomposition intended to be reinforced by a metal reinforcement accordingto the invention no longer requires the use in its formulation of metalsalts such as cobalt salts.

The first layer or sheath covering the or each reinforcing yarn isformed by a thermoplastic polymer having by definition a positive Tg(Tg₁), preferably above +20° C. and more preferably above +30° C.Moreover, the melting point (Tm) of this thermoplastic polymer ispreferably above 100° C., more preferably above 150° C. and especiallyabove 200° C.

This thermoplastic polymer is preferably chosen within the groupconsisting of polyamides, polyesters and polyimides, more particularlyfrom the group formed by aliphatic polyamides and polyesters. Amongpolyesters, mention may for example be made of PET (polyethyleneterephthalate), PEN (polyethylene naphthalate), PBT (polybutyleneterephthalate), PBN (polybutylene naphthalate), PPT (polypropyleneterephthalate), and PPN (polypropylene naphthalate). Among aliphaticpolyamides, mention may in particular be made of the polyamides 4,6, 6,6,6, 11 and 12. This thermoplastic polymer is preferably an aliphaticpolyamide, more preferably a 6,6 polyamide.

The second layer or sheath covering the first layer is formed by anunsaturated thermoplastic stirene elastomer having by definition anegative Tg (Tg₂), preferably below −20° C. and more preferably below−30° C. Thus, and according to a preferred embodiment of the invention,the difference in glass transition temperatures (Tg₁-Tg₂) between thethermoplastic polymer and the unsaturated TPS elastomer is greater than40° C., more preferably greater than 60° C.

It will be recalled here that TPS (thermoplastic stirene) elastomers arethermoplastic elastomers in the form of stirene-based block copolymers.These thermoplastic elastomers, having an intermediate structure betweenthermoplastic polymers and elastomers, are made up, as is known, frompolystirene hard sequences linked by elastomer soft sequences, forexample polybutadiene, polyisoprene or poly(ethylene/butylene)sequences.

This is why, as is known, TPS copolymers are generally characterized bythe presence of two glass transition peaks, the first (lower, negativetemperature, corresponding to Tg₂) peak relating to the elastomer blockof the TPS copolymer while the second (higher, positive temperature,typically at around 80° C.) peak relating to the thermoplastic (stireneblock) part of the TPS copolymer.

These TPS elastomers are often tri-block elastomers with two hardsegments linked by a soft segment. The hard and soft segments may bearranged in a linear fashion, or in a star or branched configuration.These TPS elastomers may also be di-block elastomers with a single hardsegment linked to a soft segment. Typically, each of these segments orblocks contains a minimum of more than 5, generally more than 10, baseunits (for example stirene units and isoprene units in the case of astirene/isoprene/stirene block copolymer).

This being reminded, one essential feature of the TPS elastomer used inthe composite reinforcement of the invention is the fact that it isunsaturated. By the expression “TPS elastomer” is understood bydefinition, and as is well known, a TPS elastomer that containsethylenically unsaturated groups, i.e. it contains carbon-carbon doublebonds (whether conjugated or not). Conversely, a saturated TPS elastomeris of course a TPS elastomer that contains no such double bonds.

Preferably, the unsaturated elastomer is a copolymer comprising stirene(i.e. polystirene) blocks and diene (i.e. polydiene) blocks, especiallyisoprene (polyisoprene) or butadiene (polybutadiene) blocks. Such anelastomer is chosen in particular within the group consisting ofstirene/butadiene (SB), stirene/isoprene (SI),stirene/butadiene/butylene (SBB), stirene/ butadiene/isoprene (SBI),stirene/butadiene/stirene (SBS), stirene/butadiene/butylene/ stirene(SBBS), stirene/isoprene/stirene (SIS),stirene/butadiene/isoprene/stirene (SBIS) block copolymers and blends ofthese copolymers.

More preferably, this unsaturated elastomer is a copolymer of thetri-block type, chosen within the group consisting ofstirene/butadiene/stirene (SBS), stirene/butadiene/butylene/stirene(SBBS), stirene/isoprene/stirene (SIS),stirene/butadiene/isoprene/stirene (SBIS) block copolymers and blends ofthese copolymers.

According to another preferred embodiment of the invention, the stirenecontent in the unsaturated TPS elastomer is between 5 and 50%. Outsidethe range indicated, there is a risk of the intended technical effect,namely an adhesion compromise with respect, on the one hand, to thelayer of the thermoplastic polymer and, on the other hand, to the dieneelastomer to which the reinforcement is moreover intended, no longerbeing optimal. For these reasons, the stirene content is more preferablybetween 10 and 40%.

The number-average molecular weight (M_(n)) of the TPS elastomer ispreferably between 5000 and 500,000 g/mol, more preferably between 7000and 450,000. The number-average molecular weight (M_(n)) of the TPSelastomers is determined, in a known manner, by SEC (steric exclusionchromatography). The specimen is firstly dissolved in tetrahydrofuranwith a concentration of about 1 g/l and then the solution is filtered ona filter of 0.45 μm porosity before injection. The apparatus used is aWATERS Alliance chromatograph. The elution solvent is tetrahydrofuran,the flow rate is 0.7 ml/min, the temperature of the system is 35° C. andthe analysis time is 90 min. A set of four WATERS “STYRAGEL” columns (anHMW7 column, an HMW6E column and two HT6E columns) are used in series.The injected volume of the polymer specimen solution is 100 μl. Thedetector is a WATERS 2410 differential refractometer and its associatedsoftware, for handling the chromatograph data, is the WATERS MILLENIUMsystem. The calculated average molecular weights are relative to acalibration curve obtained with polystirene standards.

Unsaturated TPS elastomers, such as for example SBS, SBBS, SIS or SBIS,are well known and commercially available, for example from the companyKraton under the name “Kraton D” (e.g. the products D1161, D1118, D1116,D1163 in the case of SIS and SBS elastomers), from the company Dynasolunder the name “Calprene” (e.g. the products C405, C411 and C412 in thecase of SBS elastomers) or else from the company Asahi under the name“Tuftec” (e.g. the product P1500 in the case of an SBBS elastomer).

The glass transition temperature of the above thermoplastic polymers(Tg₁ and Tg₂) is measured, in a known manner, by DSC (DifferentialScanning calorimetry), for example and except different indicationsspecified in the present application, according to the ASTM D3418 (1999)Standard.

FIG. 1 appended hereto shows very schematically (without being drawn toa specific scale), in cross section, a first example of a compositereinforcement according to the invention. This composite reinforcementdenoted by R-1 consists of a reinforcing thread (10) consisting of aunitary filament or monofilament having a relatively large diameter (forexample between 0.10 and 0.50 mm), for example made of carbon steel,which is covered with a first layer (11) of a thermoplastic polymerhaving a positive glass transition temperature (Tg₁), for example madeof a polyamide or a polyester, the minimum thickness of which is denotedby E_(m1) in FIG. 1. A second layer (12) of an unsaturated TPSelastomer, for example SBS, SBBS, SIS or SBIS, having a negative glasstransition temperature (Tg₂), covers the first layer (11) and a has aminimum thickness denoted by E_(m2) in FIG. 1.

FIG. 2 shows schematically, in cross section, a second example of acomposite reinforcement according to the invention. This compositereinforcement denoted R-2 consists of a reinforcing thread (20)consisting in fact of two unitary filaments or monofilaments (20 a, 20b) of relatively large diameter (for example between 0.10 and 0.50 mm)twisted or cabled together, for example made of carbon steel. Thereinforcing thread (20) is covered in a first layer (21) of athermoplastic polymer having a positive glass transition temperature(Tg₁), for example made of 6,6 polyamide or a polyester, with a minimumthickness E_(m1). A second layer (22) of an unsaturated TPS elastomer,for example SBS or SIS, having a negative glass transition temperature(Tg₂), with a minimum thickness of E_(m2) covers the first layer (21).

FIG. 3 shows schematically, in cross section, another example of acomposite reinforcement according to the invention. This compositereinforcement denoted by R-3 consists of three reinforcing threads (30)each consisting of two monofilaments (30 a, 30 b) of relatively largediameter (for example between 0.10 and 0.50 mm) twisted or cabledtogether, for example made or steel or carbon. The assembly formed byfor example the three aligned reinforcing threads (30) is covered with afirst layer (31) of a thermoplastic polymer having a positive glasstransition temperature (Tg₁), for example a polyamide or a polyester. Asecond layer (32) of an unsaturated TPS elastomer, for example SBS,SBBS, SIS or SBIS, having a negative glass transition temperature (Tg₂)covers the first layer (31).

FIG. 4 shows schematically, again in cross section, another example of acomposite reinforcement according to the invention. This compositereinforcement R-4 comprises a reinforcing thread (40) consisting of asteel cord of 1+6 construction, with a central wire or core wire (41 a)and six filaments (41 b) of the same diameter that are wound together ina helix around the central wire. This reinforcing thread or cord (40) iscovered with a first layer (42) of a 6,6 polyamide which is itselfcovered with a second layer (43) of an SBS elastomer.

In the composite reinforcements according to the invention, such asthose shown schematically for example in the aforementioned FIGS. 1 to4, the minimum thickness of the two layers (E_(m1) and E_(m2)) may varyvery widely depending on the particular production conditions of theinvention.

The minimum thickness E_(m1) of the first layer is preferably between 1μm and 2 mm, more preferably between 10 μm and 1 mm.

According to particular embodiments of the invention, the minimumthickness E_(m2) of the second layer may be of the same order ofmagnitude as that of the first layer (in the case of a thick secondlayer with a thickness for example between 1 μm and 2 mm, in particularbetween 10 μm and 1 mm), or else may be appreciably different.

According to another particular embodiment, the second layer could forexample be formed by a thin or ultra-thin adhesive layer deposited, forexample, not by extrusion but by a coating or spraying technique, oranother thin or ultra-thin deposition technique, for example with athickness between 0.02 μm and 1 μm, in particular between 0.05 μm and0.5 μm.

If several reinforcing threads are used, the first and second layers maybe deposited individually on each of the reinforcing threads (as areminder, these reinforcing threads may or may not be unitary), asillustrated for example in FIGS. 1, 2 and 4 commented upon above.However, the first and second layers may also be deposited collectivelyon several reinforcing threads appropriately arranged, for examplealigned along a main direction, as illustrated for example in FIG. 3.

The composite reinforcement of the invention is produced by a specificprocess comprising at least the following steps:

-   -   during a first step, initially at least one reinforcing thread        is firstly covered by the first layer of thermoplastic polymer        having a positive glass transition temperature;    -   next, during a second step, a second layer of the unsaturated        TPS elastomer, having a negative glass transition temperature,        is deposited on the reinforcing thread(s) thus covered; and    -   finally, the assembly is subjected to a thermo-oxidative        treatment in order to bond the two layers together.

The first two steps are carried out, in a manner known to those skilledin the art, continuously in line or otherwise. For example, these stepssimply consist in making the reinforcing thread pass through dies ofsuitable diameter in extrusion heads heated to appropriate temperatures.

According to a preferred embodiment, the reinforcing thread or threadsare preheated, for example by induction heating or by IR radiation,before passing into the respective extrusion heads. On exiting eachextrusion head, the reinforcing thread or threads thus sheathed are thencooled sufficiently for the respective polymer layer to solidify, forexample using cold air or another gas, or by the thread(s) passingthrough a water bath followed by a drying step.

The next step consists of a thermo-oxidative treatment. The term“thermo-oxidative treatment” is understood by definition to mean a heattreatment in the presence of oxygen, for example the oxygen in the air.Without a thermo-oxidative treatment, it has proved impossible to obtainacceptable adhesion of the TPS second layer to the thermoplastic polymerfirst layer—a vacuum heat treatment for example has proved to becompletely ineffective.

As an example, a reinforcing thread with a diameter of about 0.6 mm, forexample a metal cord consisting simply of two individual monofilamentsof 0.3 mm diameter twisted together (as for example illustrated in FIG.2) is covered with a 6,6 polyamide first layer having a maximumthickness equal to about 0.4 mm, in order to obtain a sheathedreinforcing thread having a total diameter of about 1 mm, on anextrusion/sheathing line comprising two dies, a first die (counter-dieor upstream die) having a diameter equal to about 0.65 mm and a seconddie (or downstream die) having a diameter equal to about 0.95 mm, bothdies being placed in an extrusion head heated to about 300° C. Thepolyamide, which melts at a temperature of 290° C. in the extruder, thuscovers the reinforcing thread on passing through the sheathing head, ata thread run speed typically several tens of m/min for an extrusion pumprate typically of several tens of cm³/min. On exiting this firstsheathing die, the thread may be immersed in a cooling tank filled withcold water, in order for the polyamide to solidify and set in itsamorphous state, and then dry, for example by heating the take-up reelin an oven.

The thread thus covered with polyamide is then covered with theunsaturated TPS elastomer, according to one embodiment adapted to theintended thickness for the second layer.

As a first example, if the intended thickness of the second layer isabout 0.1 mm, the thread covered with 6,6 polyamide may be passed backthrough an extrusion-sheathing line in which the sheathing head isheated to a temperature of 200° C. and fitted with a first counter-die1.1 mm in diameter and a second die 1.2 mm in diameter. The unsaturatedTPS elastomer, heated for example to a temperature of about 180° C. inthe extruder, thus covers the thread, by passing through the sheathinghead, typically at a run speed of a few m/min to a few tens of m/min foran extrusion pump throughput typically of a few cm³/min to a few tens ofcm³/min.

As a second example, if the intended thickness of the second layer isvery substantially smaller, for example equal to a few tens of ananometre, the thread covered with 6,6 polyamide passes, for example ata speed of a few m/min or tens of m/min, and over a length of several cmor tens of cm, between two wool baize elements pressed by a mass of 1 kgand continuously imbibed with the TPS elastomer diluted in anappropriate solvent (for example SBS diluted to 5% in toluene), so as inthis way to cover all of it with an ultra-thin layer of TPS elastomer(SBS).

For the two successive sheathing steps described above, the cord(reinforcing thread) is advantageously preheated, for example by passingthrough an HF generator or through a heating tunnel, before passing intothe extrusion heads.

After the second operation, i.e. directly on leaving heating head or thecoating bath that were described above, depending on the particularembodiments of the invention, the composite thread passes through atunnel oven, for example several metres in length, in order to undergotherein a heat treatment in air.

The treatment temperature is for example between 150° C. and 300° C.,for treatment times of a few seconds to a few minutes depending on thecase, it being understood that the duration of the treatment will beshorter the higher the temperature and that the heat treatmentnecessarily must not lead to the thermoplastic materials used remeltingor even excessively softening.

The composite reinforcement of the invention thus completed isadvantageously cooled, for example in air, so as to avoid undesirablesticking problems while it is being wound onto the final take-up reel.

A person skilled in the art will know how to adjust the temperature andthe duration of the treatment according to the particular operatingconditions of the invention, especially according to the exact nature ofthe composite reinforcement manufactured, particularly according towhether the treatment is on monofilaments taken individually, cordsconsisting of several monofilaments or groups of such monofilaments orcords, such as strips.

In particular, a person skilled in the art will have the advantage ofvarying the treatment temperature and treatment time so as to find, bysuccessive approximations, the operating conditions giving the bestadhesion results for each particular embodiment of the invention.

The steps of the process according to the invention that have beendescribed above may advantageously be supplemented with a finaltreatment for three-dimensionally crosslinking the reinforcement, moreprecisely its TPS elastomer second layer, in order to further increasethe intrinsic cohesion thereof, especially if this compositereinforcement is intended for being eventually used at a relatively hightemperature, typically above 100° C.

This crosslinking may be carried out by any known means, for example byphysical crosslinking means such as ion or electron bombardment, or bychemical crosslinking means, for example by incorporating a crosslinkingagent (e.g. linseed oil) into the TPS elastomer, for example while it isbeing extruded, or else by incorporating a vulcanizing (i.e.sulphur-based) system into the TPS elastomer.

Crosslinking may also take place, while the tires (or more generallyrubber articles) that the composite reinforcement of the invention isintended to reinforce, by means of the intrinsic crosslinking systempresent in the diene rubber compositions used for making such tires (orarticles) and coming into contact with the composite reinforcement ofthe invention.

The composite reinforcement of the invention can be used directly, thatis to say without requiring any additional adhesive system, asreinforcing element for a diene rubber matrix, for example in a tire.Advantageously, it may be used to reinforce tires for all types ofvehicle, in particular for passenger vehicles or industrial vehiclessuch as heavy vehicles.

As an example, FIG. 5 appended hereto shows very schematically (withoutbeing drawn to a specific scale) a radial section through a tireaccording to the invention for a passenger vehicle.

This tire 1 comprises a crown 2 reinforced by a crown reinforcement orbelt 6, two sidewalls 3 and two beads 4, each of these beads 4 beingreinforced with a bead wire 5. The crown 2 is surmounted by a tread (notshown in this schematic figure). A carcass reinforcement 7 is woundaround the two bead wires 5 in each bead 4, the upturn 8 of thisreinforcement 7 lying for example towards the outside of the tire 1,which here is shown fitted onto its rim 9. The carcass reinforcement 7consists, as is known per se of at least one ply reinforced by cords,called “radial” cords, for example textile or metal cords, that is tosay that these cords are arranged practically parallel to one anotherand extend from one bead to the other so as to make an angle of between80° and 90° with the median circumferential plane (the planeperpendicular to the rotation axis of the tire, which is located atmid-distance from the two beads 4 and passes through the middle of thecrown reinforcement 6).

This tire 1 of the invention has for example the essential feature thatat least one of the crown or carcass reinforcements thereof comprises acomposite reinforcement according to the invention. According to anotherpossible embodiment of the invention, it is the bead wires 5 that couldbe made from a composite reinforcement according to the invention.

EMBODIMENT OF THE INVENTION

Trial 1—Composite Reinforcement Manufacture

Composite reinforcements, according to or not according to theinvention, were firstly manufactured in the following manner. Thestarting reinforcing thread was a steel cord for tires, made of standardsteel (having a carbon content of 0.7% by weight), in 1×2 constructionconsisting of two individual threads or monofilaments 0.30 mm indiameter twisted together with a helix pitch of 10 mm. Cord diameter was0.6 mm.

This cord was covered with 6,6 polyamide (ZYTEL E40 NC010 from thecompany DuPont de Nemours; melting point T_(m) equal to about 260° C.)was performed on an extrusion-sheathing line by passing it through anextrusion head heated to a temperature of 300° C. and comprising twodies—an upstream die 0.63 mm in diameter and a downstream die 0.92 mm indiameter. The polyamide heated to a temperature of about 290° C. in theextruder (pump rate of 20 cm³/min) thus covered the thread (preheated toabout 280-290° C. by passing it through an HF generator) running at aspeed of 30 m/min. On leaving the sheathing head, the compositereinforcement obtained was continuously run through a cooling tankfilled with water at 5° C., in order for the polyamide to solidify inits amorphous state, before being dried using an air nozzle.

This stage of the manufacture resulting in a control compositereinforcement (therefore not in accordance with the invention)consisting of the initial steel cord sheathed only with its polyamidefirst layer. This control composite reinforcement (denoted by R-5) had atotal diameter (i.e. once sheathed) of about 1.0 mm.

Next, during a second step, a second layer of an SBS thermoplasticelastomer (D1118 from the company Kraton), the intended minimumthickness (E_(m2)) of which about 0.1 mm, was deposited on the cord thussheathed in the following manner: the cord covered with 6,6 polyamide(preheated to about 100° C. by passing it through a heating tunnel) waspassed back through a second extrusion-sheathing line in which thesheathing head was heated to a temperature of 200° C. and fitted with anupstream die 1.1 mm in diameter and with a downstream die 1.2 mm indiameter. The SBS elastomer, heated to a temperature of 180° C. in theextruder, thus covered the cord, by passing through the sheathing head(pump rate: 2 cm³/min) at a run speed of 3 m/min.

The glass transition temperatures Tg₁ and Tg₂ of the two polymers usedabove were equal to about +50° C. and −95° C. respectively (822-2 DSCinstrument from Mettler Toledo; helium atmosphere; specimens preheatedfrom room temperature (20° C.) to 100° C. (at 20° C./min) and thenrapidly cooled down to −140° C., before finally recording the DSC curvefrom −140° C. to +250° C. at 20° C./min).

After this second sheathing operation, the assembly (doubly-sheathedcomposite reinforcement) underwent a thermo-oxidative treatment for atime of about 100 s, by passing it through a tunnel oven at 3 m/min inan ambient atmosphere, heated to a temperature of 270° C. This finalstage of the manufacture resulted in a composite reinforcement accordingto the invention, consisting of the initial steel cord sheathed with itspolyamide first layer and with its SBS elastomer second layer. Thecomposite reinforcement according to the invention produced in this way(the reinforcement R-2 as shown schematically in FIG. 2) had a finaltotal diameter of about 1.1 mm.

To determine the best operating conditions for the thermo-oxidativetreatment in the above trial, a range of temperatures from 160° C. to280° C., for four treatment times (50 s, 100 s, 200 s and 400 s), wasexamined beforehand.

Trial 2—Adhesion Tests

The quality of the bond between the rubber and the compositereinforcements manufactured above was then assessed by a test in whichthe force needed to extract the reinforcements from a vulcanized rubbercomposition, also called a vulcanizate, was measured. This rubbercomposition was a conventional composition used for the calendering ofmetal tire belt plies, based on natural rubber, carbon black andstandard additives.

The vulcanizate was a rubber block consisting of two sheaths measuring200 mm by 4.5 mm and with a thickness of 3.5 mm, applied against eachother before curing (the thickness of the resulting block was then 7mm). It was during the conduction of this block that the compositereinforcements (15 strands in total) were imprisoned between the tworubber sheets in the uncured state, an equal distance apart and with oneend of each composite reinforcement projecting on either side of thesesheets an amount sufficient for the subsequent tensile test. The blockcontaining the reinforcements was then placed in a suitable mould andthen cured under pressure. The curing temperature and the curing time,left to the discretion of a person skilled in the art, were adapted tothe intended test conditions. For example, in the present case, theblock was cured at 160° C. for 15 minutes under a pressure of 16 bar.

After being cured, the specimen, thus consisting of the vulcanized blockand the 15 reinforcements, was placed between the jaws of a suitabletensile testing machine so as to pull each reinforcement individuallyout of the rubber, at a given pull rate and a given temperature (forexample, in the present case, at 50 mm/min and 20° C. respectively). Theadhesion levels were characterized by measuring the pull-out force(denoted by F_(max)) for pulling the reinforcements out of the specimen(this being an average over 15 tensile tests).

It was found that the composite reinforcement of the invention, despitethe fact that it contains no RFL adhesive (or any other adhesive), had ahigh pull-out force F_(max) equal to about 60% of the control pull-outforce measured on the nylon-sheathed control composite reinforcement(R-5) and bonded using a conventional RFL adhesive.

Under the same conditions, the control composite reinforcement (R-5)sheathed with nylon but containing no RFL adhesive (or any otheradhesive), showed no adhesion to the rubber (practically zero pull-outforce).

The result obtained on the composite reinforcement of the invention is,to a person skilled in the art, both remarkable and unexpected,especially as, before the thermo-oxidative treatment, the pull-out forceof the same composite reinforcement proved too to be practically equalto zero.

Additional trials have also demonstrated that the use of the unsaturatedTPS elastomer (SBS), not as the second sheathing layer but as adhesiveincorporated into the rubber calendering composition (and thereforealways in contact with the composite reinforcement) provided no adhesivecontribution.

Consequently, the composite reinforcement of the invention, owing to itsself-adhesive character, constitutes a useful and entirely crediblealternative, on account of the adhesion levels obtained, to thecomposite reinforcements of the prior art that are sheathed with athermoplastic material such as a polyamide or polyester which require,as is known, the use of an RFL adhesive to ensure that they adhere tothe rubber.

The invention claimed is:
 1. A composite reinforcement comprising: atleast one reinforcing thread; a first layer of a thermoplastic polymer,the glass transition temperature of which is positive, covering saidthread; and a second layer comprising an unsaturated thermoplasticstyrene elastomer, the glass transition temperature of which isnegative, covering the first layer; wherein the unsaturatedthermoplastic styrene elastomer of the second layer is crosslinked. 2.The reinforcement according to claim 1, wherein the glass transitiontemperature of the thermoplastic polymer is above +20° C.
 3. Thereinforcement according to claim 1, wherein the glass transitiontemperature of the unsaturated thermoplastic styrene elastomer is below−20° C.
 4. The reinforcement according to claim 1, wherein thedifference in glass transition temperature between the thermoplasticpolymer and the unsaturated thermoplastic styrene elastomer is greaterthan 60° C.
 5. The reinforcement according to claim 1, wherein thethermoplastic polymer is an aliphatic polyamide or a polyester.
 6. Thereinforcement according to claim 5, wherein the thermoplastic polymer isa 6,6 polyamide.
 7. The reinforcement according to claim 1, wherein thethermoplastic elastomer is a copolymer comprising styrene blocks anddiene blocks.
 8. The reinforcement according to claim 7, wherein thediene blocks are isoprene or butadiene blocks.
 9. The reinforcementaccording to claim 8, wherein the thermoplastic elastomer is chosenwithin the group consisting of styrene/butadiene (SB), styrene/isoprene(SI), styrene/butadiene/ butylene (SBB), styrene/butadiene/isoprene(SBI), styrene/butadiene/styrene (SBS),styrene/butadiene/butylene/styrene (SBBS), styrene/isoprene/styrene(SIS), styrene/butadiene/isoprene/styrene (SBIS) block copolymers andblends of these copolymers.
 10. The reinforcement according to claim 9,wherein the thermoplastic elastomer is an SBS or SIS copolymer.
 11. Thereinforcement according to claim 1, wherein the thermoplastic elastomercomprises between 5 and 50% styrene by weight.
 12. The reinforcementaccording to claim 1, wherein the minimum thickness of the first layeris between 1 μm and 2 μm.
 13. The reinforcement according to claim 1,wherein the minimum thickness of the second layer is between 1 μm and 2μm.
 14. The reinforcement according to claim 1, wherein the minimumthickness of the second layer lies in the range from 0.02 μm to 1 μm.15. The reinforcement according to claim 1, wherein the reinforcingthread is a metal wire.
 16. The reinforcement according to claim 15,wherein the metal wire is a carbon steel wire.
 17. A reinforcementarticle or semi-finished product comprising the composite reinforcementaccording to claim
 1. 18. A pneumatic tire comprising a compositereinforcement according to claim
 1. 19. A process for manufacturing acomposite reinforcement according to claim 1, capable of adhering to adiene rubber matrix, comprising the steps of: the reinforcing thread iscovered by a layer of the thermoplastic polymer having a positive glasstransition temperature; a second layer comprising the unsaturatedthermoplastic styrene elastomer, having a negative glass transitiontemperature, is deposited on the thread thus covered; and the assemblyundergoes a thermo-oxidative treatment in order to bond the two layerstogether.
 20. The process according to claim 19, wherein the glasstransition temperature of the thermoplastic polymer is above +20° C. 21.The process according to claim 19, wherein the glass transitiontemperature of the unsaturated thermoplastic styrene elastomer is below−20° C.
 22. The process according to claim 19, wherein the difference inglass transition temperature between the thermoplastic polymer and theunsaturated thermoplastic styrene elastomer is greater than 60° C. 23.The process according to claim 19, wherein the thermoplastic polymer isan aliphatic polyamide or a polyester.
 24. The process according toclaim 19, wherein the thermoplastic elastomer is a copolymer comprisingstyrene blocks and diene blocks.
 25. The process according to claim 24,wherein the thermoplastic elastomer is chosen within the groupconsisting of styrene/butadiene (SB), styrene/isoprene (SI),styrene/butadiene/ butylene (SBB), styrene/butadiene/isoprene (SBI),styrene/butadiene/styrene (SBS), styrene/ butadiene/butylene/styrene(SBBS), styrene/isoprene/styrene (SIS), styrene/butadiene/isoprene/styrene (SBIS) block copolymers and blends of these copolymers.26. The process according to claim 25, wherein the thermoplasticelastomer is an SBS or SIS copolymer.
 27. The process according to claim25, wherein the thermoplastic elastomer is an SBS copolymer.